The present invention relates to synchronous dynamolectric machines and, more particularly, to large synchronous dynamoelectric machines employing pole-face or damping windings shorted by amortisseur rings.
A synchronous dynamoelectric machine, either motor or generator, is capable of developing torque or generating electricity only when its rotor rotates at the same speed as the rotating magnetic field in its air gap with the phase angle of the magnetic field of the rotor separated from the phase angle of the rotating electrical field by less than about 180 degrees and preferably by less than 90 degrees. This speed is called the synchronous speed. The exact value of the synchronous speed depends on the line frequency of the AC supply and on the number of poles in the machine. In a two-pole machine operating with 60 Hz power, for example, the synchronous speed is 3600 RPM.
In order to accelerate a motor rotor from a standstill to synchronous speed it is customary in one type construction to embed amortisseur bars, or pole-face windings, in the pole faces of the rotor with shorting rings or amortisseur rings or segments thereof shorting the ends of the bars together. When the stator is excited with AC, the amortisseur windings interact with the rotating magnetic field in the same fashion as an induction motor to accelerate the rotor to a speed close to synchronous speed. The rotor poles are then excited with DC and, if the load is not too great, the rotor is pulled into synchronous rotation with the rotating magnetic field. As long as synchronous speed is maintained, the amortisseur bars remain substantially quiescent since there is, by definition, no sustained rotor slip when the rotor rotates at synchronous speed and there is thus little or no current in the amortisseur bars.
For manufacturing convenience, the amortisseur rings are made in sectors, one sector at each end being associated with one of the motor poles. That is, for a six-pole motor, the amortisseur rings at each end are made in six sectors which are joined together to form the completed amortisseur rings during final assembly of the rotor. During the relatively short period while the rotor is being started, a very high value of current flows in the amortisseur bars. This produces substantial heating and thermal expansion of the amortisseur bars. Once synchronous speed is attained, the heating ceases and the amortisseur bars begin to cool. It is known that the heating of amortisseur bars is not uniform. The bar at the leading edge of each pole piece becomes much hotter than the conductor bars further back from the leading edge.
In order to keep the differences in expansion of the amortisseur bars due to differential heating from generating excessive forces in the amortisseur rings it is conventional to rigidly affix only the bars at the extreme ends of each amortisseur sector in holes in the respective sectors and to leave the ends of the conductor bars between these end conductor bars free to move axially within loosely fitting holes in the sectors. The fixed conductor bars are mechanically and electrically connected to their sectors by brazing or equivalent means. Current is conducted between the free conductor bars and their sectors by flexible conducting braid.
As the sizes of synchronous motors has increased beyond about 20,000 horsepower, there is a tendency, under certain heavy-duty cycles, for cracks to develop in the outer surface of the amortisseur rings radially outward from the loosely fitting holes aligned with the axis of the loosely fitting holes. This type of cracking is not observed adjacent the fixed conductor bars. Although I do not intend to limit the scope of my invention to any particular causal theory, it is believed that a combination of heating and centrifugal force contributes to the development of such cracking.
Centrifugal force urges the ends of the amortisseur bars outward into axially directed lines of contact with the radially outward extremity of the loosely fitting holes in the amortisseur ring. It is believed that, during starting, a substantial current flows between the end of an amortisseur bar and the amortisseur ring through the poorly conducting line of contact between them. This current flow generates heat which, combined with the heat generated by starting current in the amortisseur bar and the urging of centrifugal force, exceeds the ability of the thin section of the amortisseur ring between the loosely fitting hole and its radially outer surface to withstand. This heating and application of centrifugal force is cyclical, occurring each time the motor is started, and is usually immediately followed by cooling. This thermal and stress cycling appears to be the root of the observed cracking.
The occurrence of cracking in the outer extremities of the amortisseur rings adjacent the free amortisseur bars does not constitute a major safety or operational hazard since the centers of the conductor bars are usually firmly secured in the pole pieces and the most severe of the observed cracks are quite narrow compared to the diameters of the conductor bars. Such cracking cannot, however, be tolerated, since any failure in a dynamoelectric machine may lead to other unforeseeable effects which could eventually damage the machine or cause a serious failure.
Although the above description of the background and the ensuing description of the present invention are directed concretely toward use in amortisseur windings of a motor, the disclosure herein applies equally to amortisseur windings of generators.